In diesel engines, fuel injected through a fuel injection valve into a cylinder is ignited when a predetermined period of time has elapsed after the injection (this is referred to as an ignition delay). In order to improve the output and emission performances of diesel engines, control devices are widely employed to control the execution modes of engine control, such as timing and amount of fuel injection, upon consideration of such an ignition delay.
Diesel engines undergo a longer ignition delay as fuel having a lower cetane number is used. For this reason, even if the execution modes of engine control may have been set before shipping of diesel engines, for example, assuming the situation where fuel having a standard cetane number is used, fuel having a relatively low cetane number such as fuel for winter use, when supplied to a fuel tank, would have delayed ignition timing and undergo a poor combustion state, which may result in a misfire.
To avoid such an inconvenience, it is desirable to correct the execution modes of engine control based on the actual cetane number of fuel injected into a cylinder. It is then necessary to accurately estimate the cetane number of the fuel to make such a correction in a favorable manner.
Patent Document 1 has proposed a conventional device for injecting a small amount of fuel through a fuel injection valve and detecting an index value of an engine torque produced as a result of the fuel injection to estimate the cetane number of the fuel based on the index value. Focusing on the fact that the engine torque produced as a result of the injection of a predetermined amount of fuel varies depending on the cetane number of the fuel, the device is configured to estimate the cetane number of the fuel based on an index value of an engine torque produced as a result of the fuel injection.